Roofing shingles are usually made by taking a continuous base sheet of material (e.g. organic felt, fiberglass mat or the like), saturating the base sheet in a base asphalt, covering it with a coating asphalt, and then embedding granules on the top side of the coated sheet. The granules protect the asphalt from breaking down by oxidation caused by ultraviolet rays. The finished sheet is then cut into lanes and then into desired lengths for shingles.
It is known to provide additional decoration for each shingle by providing a patterned appearance on its exposed surface. Commonly the pattern takes the form of a patch-like appearance, with a sequence of areas of granules of one color separated by areas of granules of a different color. Such an appearance is shown for example in U.S. design Pat. No. D309,027.
When a patch-like decorative appearance is created, it is desirable to have a sharp line of demarcation between the color in one area and the color in an adjacent area. Unfortunately, it is extremely difficult to achieve a sharp line of demarcation. The problem is that the granules which form the exposed surface of the shingle are normally dropped by a blender on a base sheet which is travelling at 500 to 600 feet per minute, or more than 8 feet per second. It is difficult to turn off the flow of granules of one color and to start the flow of granules of another color in a sufficiently short time to produce sharply demarcated edges between the two adjacent colors. For example, if it takes 0.01 seconds to start or stop dispensing granules, during this time the sheet will have travelled about one inch, so the transition between areas of different colors would be about one inch long.
Because of this problem, it has been common practice in the past to create the desired patch-like appearance by coating the entire surface of the base sheet with a first layer of granules, and then applying patches of asphalt and granules as a second layer over the first layer of granules. This has the advantage that the granules applied to the patches of asphalt adhere only to those patches, providing sharp transitions. U.S. Pat. No. 4,352,837 (Kopenhaver) and U.S. Pat. No. 5,186,980 (Koschitzky) both disclose methods of applying patches of asphalt and granules as a second layer to a first uniform layer of granules.
However applying second layers of asphalt and granules to the shingle has disadvantages in terms of increased cost. In addition the extra thickness can result in decreased flexibility of the shingle. Therefore it would be desirable to create the appearance of sharply demarcated areas of granules without the need for applying extra layers of asphalt and granules.